1. Field of the Invention
The present invention relates to a direct conversion receiver and, more particularly, to a discrete-time direct conversion receiver having excellent adaptability and applied to a wireless communication terminal.
2. Description of the Related Art
In general, a charge sampling receiver includes a sampler performing frequency down-conversion and signal sampling. The sampler performs signal sampling by using voltage sampling or charge sampling.
A charge sampling mixer has aliasing and noise folding elimination characteristics installed therein, having an excellent effect as a sampler compared with a voltage sampling mixer.
A conversion gain of a direct conversion down-sampling mixer is represented by Equation 1 shown below:
                              C          ⁢                                          ⁢          G                =                                            v              IF                                      v              RF                                =                                    2              π                        ⁢                                          G                m                                                              f                  S                                ·                                  C                  S                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Here, Gm is transconductance of a transconductor, fs is a sampling frequency, and Cs is capacitance of a sampling capacitor.
According to Equation 1, the conversion gain of an aliasing elimination filter in the form of a first-order sinc filter installed in the charge sampling mixer has frequency dependency. Thus, because the conversion gain of the anti-aliasing sinc filter installed in the charge sampling mixer has frequency dependency characteristics, it is difficult to apply the related art charge sampling receiver to a broadband application such as a digital TV tuner having a frequency band ranging from tens MHz to a few GHz.
Another drawback of the charge sampling mixer is linearity characteristics. In the broadband application, linearity characteristics are one of the most important specifications because the linearity characteristics are related to blocking interferences. The swing of outputs from the transconductance amplifying stage and the sampling mixer is significantly limited due to the linearity characteristics of the receiver.